The present invention relates in general to a communications satellite payload with digital channelization and digital beam-forming. More particularly, the invention relates to a completely autonomous failure recovery scheme, providing self diagnosis and self healing, for faulty signal processing chains in a satellite payload. Such scheme significantly enhances the reliability of the satellite payload.
FIG. 1 illustrates a communications satellite payload with digital channelization and digital beam-forming. Frequency multiplexed traffic from a few large gateway stations is received by global antenna 1. This traffic is then routed through the upper branch of the figure, demultiplexed into discrete bands, processed, by a respective chain of conventional filter and multiplex components, and transmitted from an array antenna 2A. In order to route each channel of traffic to its appropriate destination beam, the total traffic uplinked to the global antenna 1 must be channelized into narrow frequency bands containing one or a few channels each. These bands are routed to predetermined destination beams via corresponding processing chains.
Similarly, in the return direction, the traffic received by the array antenna 2A is passed through several elements in a respective chain of conventional demultiplex and filter components on board the satellite and routed, through the lower branch of the figure, to the global antenna for transmission to the destination gateway stations. The processing that is required for the gateway receiving direction is quite similar to the processing that is conducted in the gateway transmitting direction.
The process for channelization of a gateway transmitted signal on-board the satellite consists of three operations in three separate elements. Frequency demultiplexing is conducted in demultiplexer 4, switching is conducted in switch 5, and frequency remultiplexing is conducted in remultiplexer 8. The demultiplexer 4 segments an up-link frequency band into a number of sub-bands. The switching unit 5 directs the signals in each sub-band to a corresponding processing chain having a respective input port. Finally, the frequency remultiplexer 8 in each processing chain assembles the various signals that are switched to each port into a composite signal for modulation onto a carrier and transmission from a respective array 2A.
The demultiplexed signals that are directed to a predetermined processing chain 2B are fed through a beam-forming network 6, which forms beams in the desired directions by assigning an appropriate delay in each antenna element path 2B. In conventional RF beam-forming, the required delays are implemented using microwave phase shifters, whereas in digital implementations, the effect of the delays (phase shifts) is introduced by adding a proper phase value to the baseband signal. The effect of these delays (phase shifts) is to cause the signals that are assigned to a given beam to add up coherently (phase build-up) in that beam direction. The large number of beams and antenna feed elements that are planned for future satellites makes the mass requirements of analog channelizers and beam-formers prohibitively large. The flexibility in channelization and beam-forming for those future satellites is best met with advanced digital technology.
Digital Beam Forming (DBF) is performed on a sample waveform by introducing an appropriate phase shift to each complex sample of the waveform at every antenna processing chain 2B. The phase shift that is introduced will depend on the position of the element in the array and on the desired direction of the beam. The cumulative effect of the phase shifts is to cause input samples to each processing chain to add up coherently ("phase build-up") in the desired beam direction. It is sometimes desirable to introduce some amplitude weight shifting, in addition to the phase shift, to shape the beams. In this case, the DBF operation amounts to multiplying each input sample by an appropriate complex number. Multiple beams are formed by simple superposition, since the beam-forming and channelization operations are linear.
FIG. 2 illustrates the baseline processor of FIG. 1, with four processing chains in the upper branch of the satellite communications payload. The output of an analog-to-digital converter 3 is coupled to a demultiplexer 4 whose output provides a common input for all of the processing chains 20. Although only four parallel processing chains 20 are illustrated, the total number of processing chains 20 may be much larger. Each processing chain 20 consists of digital beam forming 6, a buffer 7 and a remultiplexer 8. The output of each chain is then input to a digital to analog converter 9.
The conventional method for diagnosing failures in the processing chains 20 of a satellite payload is based on introducing an external stimulus, such as a pilot signal, and measuring the processor's response to that stimulus. The disadvantage of this method is the need for additional hardware to stimulate the processor and measure its response. In many cases, this also results in a disruption of real time operation.
Therefore, there is a need for a self diagnosis scheme, which is able to detect failures in satellite payload processing chains by capitalizing on the availability plural processing chains that are identical in hardware, and differ only in software. Further, there is a need for a self healing scheme that is able to automatically replace a processing chain, which has been diagnosed as being faulty, with a "hot" standby chain. In this environment, it is desirable to provide both self diagnosis and self healing of processing chains 20 with little added hardware and without a disruption of normal operation.